Process for insulating magnetic bodies



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CROSS REFERENce coAuNe OR 91 1mm.

Examiner 8 f tented Apr. 12, I

UNITED STATES. PATENT OFFICE CONRAD D. OWENS, OI YOB K, N. Y., ASSI GNOR TO BELL TELEPHONE LA B ORATOBIES, INCORPORATED, OF-NE'W YORK, N. Y., A CORPORATION OF NEW YORK PROCESS FOR INSULATING MAGNETIC BODIES Ho Drawing. Application filed July 1, 1931. Serial No. 548,259.

The. invention relates to magnetic bodies and particularly to methods of producing magnetic bodies of the type employing finely divided magnetic material.

, An object of the invention is to improve the magnetic and mechanical characteristics of magnetic bodies of the type employing magnetic material in finely divided form, for exairsnple, magnetic dust cores for loading co1 19 Magnetic cores made from finely divided magnetic material, so-called magnetic dust cores, are being extensively used for the loading coils which are inserted in telephone lines to. im rove their transmission characteristics. uch cores are usua constructed by subjecting the finely divide particles of the magnetic material and a non-magnetic material between the magnetic particles, to exzo tremely high "pressures in the neighborhood of 200,000 pounds per square inch; The high pressures are utilized to form the component materials into a substantially homogeneous .mass of the desired core form, and to give suflicient mechanical strength to the completed core. The non-magnetic material is of such nature as to'serve as a binder, and also as an insulation between the magnetic particles to reduce the eddy current loss in so the completed core. In the case especially where the magnetic material in the core made in the above described manner is a magnetic alloy including nickel and iron, it is desirable also to subject the core to a subsequent an- 36 'nealing heat treatment at a high temperature to remove the strains introduced in the magnetic material by the pressing operation which tend to impair the magneticproperties of the magnetic alloy in the core, and to' 0 fully develop the magnetic characteristics of e the alloy. It is desirable therefore that the insulating material have such characteristics and be applied in such manner to the dust particles that it will withstand without de-- terioration the heat treatment and mechanical operations utilized in producing the core.

A method commonly used for applying the insulating material to the magnetic particles in the rocess of making'loading coil cores is to mm the magnetic dust with the insulate ing material in powder form in a volatile liquid, such as water, and then to subject the mixture continuously to heat with constant stirring until the volatile constituents havebeen evaporated and the magnetic particles have been coated with the insulating material. Usually the magnetic dust and insulation mixtureis evaporated to dryness within a short time of the order of an hour after the volatile liquid is added thereto. The coated magnetic particles are then compressed under high pressures into the desired core form and the resultin cores are given a subsequent annealing heat treatment to improve their. magnetic characteristics.

In accordance with one embodiment of the present invention, magnetic cores made by the general process described above may be improved from a magnet standpoint if the following changes are made in the method.

of insulating. After the volatile liquid has been added to the magnetic dust and insulation mixture and the mass has been heated for a short time with continuous stirring to start to dryness. This modified treatment results in improved magnetic properties, especially a lowering of the eddy current loss in the final core.

The exact nature and advantages of the invention will be better understood from the following detailed description.

In practicing the present invention, the magnetic material for the magnetic body or core is preferably prepared from a brittle allow containing nickel and iron, with or without the addition of other constituents, and commonly known asip er nalloy. The allo may be prepared in thdm anner descri ed in United States Patent No. 1,669,649 issued May-'15, 1928, to C. P.- Beath and H. E. Heinicke, wherein the magnetic metals including nickel and iron in proper porportions depending upon the desired percentage composition of the alloy are melted together l comma 0R mm in an oxidizing atmosphere,the' resulting alloy being poured into a mold;

The ingots so obtained, while still hot, are successively passed throu h progressively reducing rolls to decrease t eir cross-section to the required size, the final roll being preferably efiected at approximately the temperature at which the alloy ceases to be malleable, after which the rolled material is quenched in water at a temperature below that at which it loses its malleability. This mechanical treatment is for the purpose of producing a fine crystalline structure in the" alloy which is desirable since. the disintegration of the material takes place at the crystal boundaries,

and consequently the smaller the-size of the crystals in the alloy material, the finer the magnetic dust which can be produced therefrom. The resultant ingots are broken into short pieces and the pieces reduced to finely divided form in any suitable manner, for example, by crushing in a rock' crusher, hammer mill or other suitable apparatus, and

pulverizing the crushed material by rolling in an attrition 'or ball mill for several hours. After rolling, the resulting dust is sieved through a 120-mesh sieve and that portion which will not passthrough the sieve is returned to the ball mill for further rolling,

this rocess bein re eated until a sufiicient P in P guantity of the e dust is obtained. The ne dust so obtainedis then annealed in a closed container at a temperature of approximately 885 C. The annealed dust is removed from the container in the form of a cake which is reduced to a powder by crushing in a rotary crusher and subsequently grindin in an attrition or ball mill. The ground ust is again sifted through a 120-mesh sieve and the dust assing through the sieve is then mixedwit a suitable insulating composition.

The method of insulating the annealed magnet dust particles in accordance with the mventlon will escribed for the case-where the insulatin com osition comprises talg, kaolin, saium sili t a water-soluble oror exam le, ta aric am an anic am E Q 3 water as disclosed in t e copen mg ap'plication of C. C. Neighbors, Serial No. 505,956,

filed December 31, 1930, although the proces of the invention is applicable as well when other insulating materials capable of with-- standing without appreciable deterioration heat treatmentat high temperatures are used.

The permalloy dust and the insulating composition preferably in the proportions specified in t e above mentioned copending application, are thoroughly mixed. Heat is, then applied to the mixture to start the process of evaporation, the heating'being continued with constant stirring-until the mixture is reduced to a plastic moist mass. The heating is then discontinued and the moist mass allowed to stand for a long period of time ranging from 4 to 24 hours before heat pnuoo nnrnnuwn is again applied and the evaporation to dryness completed to coat the magnetic particles with the insulating'material. 'In the commerical manufacture of magnetic dust cores, it is desirable, of course, for reasons of economy that the total evaporation time be reduced to the shortest possible period that will produce good results. Experience indicates that an average period of about 18 hours for the time which the moist mixture is allowed to stand without application of heat,

gives the final core sufiiciently good magnetic properties.

It is, of course, apparent that the general method of insulating, above described, may be modified to some extent within the scope of the invention. For example, similar results may be obtained by allowing the wet mixture to stand for an appreciable time before heat is applied or by applying the heat-thereto at intervals allowing it to stand for an appreciable period between the applications of heat, in which case the total time of standing the above mentioned patent application to Nei hbors, the annealing temperature used is pre erably about 565 C., but may be as high as 620 C. If the former temperature is used, the core parts are preferably raised to that temperature in about eleven minutes and maintained at such temperature for about three minutes. If higher annealing temperatures are used,-the time should, of course,'be proportionately shortened, and vice versa..

One or more core parts thus formed are't-hen stacked to form a core to which the usual toroidal winding is applied to form a loading coil, the number of core parts used in each core depending upon the electrical character istics of the circuit with which the loading coil is to be associated.

A number of loading coil cores were made "by a process similarto that described above in which the delayed evaporation? method of insulation was used for comparison with other cores of similar construct-ion made by the same process but in which the insulation process was carried out in the usual manner without delaying the evaporation to dryness Tests on thetwo lots of cores showed that the former for a given magneticpiaterial and a given insulating material had much lower eddy current losses for unit permeability body than the latter, while the hysteresis losses remamed about the same. In the case of certain cores in which the magnetic material comprised finely divided magnetic allo containing approximately We'fit'fifc e1, 17 per cent iron and 15 per cent molybdenum, and 4 per cent of a fourth metal, and the insulating material comprised the tartaric i fii ifsiiliateztek;keq n 5? "sfiEh as disclosed in the aBove fiieiitioned' Neighbors patent application, it was found that a delay of approximately eighteen hours in the evaporation of the insulation and dust mixture in the process of insulation, reduced the eddy current loss to less than one-half that of similar cores made by the standard process in which the evaporation to dryness after the liquid part of the insulation had been added to the mixture of magnetic dust and insulating material lasted only about forty-five minutes.

It is understood that the invention is not to be limited to the exact embodiments above described, but only within thescope of the appended claims. a

What is claimed is:

1. In the process of making a magnetic body comprising finely divided magnetic particles insulated from each other by insulating material, the method of insulating the magnetic particles which consists in mixing.

said magnetic particles with the insulating material in finely divided form, in a volatile liquid, allowing the mixture to stand for a time long in comparison with the time which would be required toevaporate said mixture to dryness with the continuous application of heat, and then evaporating the mixture to dryness with the application of heat to form adherent coatings of said insulating material on said particles.

2. In the process of making a magnetic comprising finely divided magnetic particles insulated from each other by insulating material, the method of insulating the magnetic particles which consists in mixin sald articles with said insulating materlal in nely divided form, in a volatile liquid, al-

lowing the mixture to stand for several hours, and then evaporating the mixture to dryness with the application of heat to form adherent coatings of said insulating material on said magnetic particles. 1

3. In the process of maln'ng a magnetic body comprising finely divided magnetic particles insulated from each other by insulating material which consists in mixing said-particles with the insulating material in powdered form in a volatile liquid, partially evaporating said mixture with the application of heat, than removing the heat and allowing the mixture to stand exposed to the atmosphere for several hours, and then applying heat to the moist mixture to evaporate it to dryness sons to form adherent coatings of the insulating material on the magnetic particles.

4. The process of producinga magnetic body having low eddy current loss from finely d1- vided magnetic material and an insulating composition comprising a water-soluble organic acid, sodium silicate, finely divided ce- 

